A non-impact printing process can be simply defined as a process which uses an electronic, electric, or optical means to produce characters as opposed to a mechanical means. Of the non-impact printing processes, there is a group of printing methods that use electrostatic techniques. Electrostatic printing can be defined as those methods which use the interaction of electrostatically charged marking particles and an electric field to control the deposition of the marking particles onto a substrate, and encompasses processes generally known as electrographic, electrophotographic, or electrostatographic printing, as well as electrostatic ink jet printing.
Electrostatography can be a term used to describe the various non-impact printing processes which involve the creation of a visible image by the attraction of charged imaging particles or marking particles to charged sites present on a substrate. Such charged sites, forming what is usually termed a latent image, can be transiently supported on photoconductors or pure dielectrics and may be rendered visible in situ or be transferred to another substrate to be developed in that location. Additionally such charged sites may be the reflection of those structured charges existing within a permanently polarised material as in the case with ferroelectrics or other electrets.
In electrostatography the imaging particles, generally known as toner, can be of the dry type or of the liquid type. Dry powder toners have many disadvantages. For example the performance of dry powder toners is very susceptible to environmental conditions, influencing, for example, charge stability, and therefore giving rise to variable image performance. Also, the large particle size of dry powder toners is a major contributing factor in not allowing the achievement of highly resolved developed images. For high speed, long run printing, cost per page is a principal consideration. In particular, the cost of fusing the image to paper or any other desired substrate significantly contributes to the running costs of such a printer. Other objections are related to the problem of dusting. Dust or fine or small particles of toner are prone to escape from the developer, and these deposit onto any surface both within and outside the printing device, causing mechanical failures within the device and environmental problems outside the device. This problem becomes severe when such printing devices are run at high speed. Other disadvantages include cost of the general maintenance of the printer and cost of the dry powder toner.
It is known that latent electrostatic images can be developed with marking particles dispersed in insulating or non-polar liquids. These dispersed materials are known as liquid toners or liquid developers. Liquid toner development systems are generally capable of very high image resolution because the toner particles can safely be much smaller, normally in the range of 0.5 to 3 μm, than dry toner particles which are normally in the range of 7 to 10 μm. Liquid toner development systems show impressive gray scale image density response to variations in image charge and achieve high levels of image density using small amounts of liquid developer. Additionally, the systems are usually inexpensive to manufacture and are very reliable, furthermore the liquid toners for these systems are operationally and chemically stable thus exhibiting a particularly long shelf-life.
Ink jet printing is a non-impact printing process that does not involve a latent image; it is a direct printing system. It is usual for an ink to be fed through a nozzle. The droplets may be produced from the nozzle either continuously in which case the method is termed continuous printing or they may be produced individually as required in which case the method is termed drop on demand printing. In continuous printing an ink is delivered through the nozzle at high pressure and the nozzle is perturbed at a substantially constant frequency which results in a stream of droplets of constant size. By applying charge to the droplets and using an electric field external to the nozzle selected droplets may be deflected in their passage to the recording surface in response to a signal effecting the electric field whereby forming a pattern on the recording surface in response to the control signal. Drop on demand printing operates by producing local pulses in the liquid in the vicinity of a small nozzle which results in a droplet of liquid being ejected from the nozzle.
In either type of jet printing the colouring material is a soluble dye combined with binders to render the printed image more permanent. The disadvantage of soluble dyes is that the printed image density is not high enough in many applications and that the dyes fade under exposure in the environment. A further disadvantage with soluble dye materials is that the quality of the printed image is dependent on the properties of the recording surface. Pigmented inks are known to produce higher density images than soluble dyes and are also more permanent. Pigments may also be used in jet printers but the production of a dense image requires a high concentration of pigment material in a liquid carrier. The high concentration of pigment material affects the droplet break-up in continuous printers and results in less uniform printing. Drop on demand printers do not have a high continuous pressure and the droplet generation is strongly dependent on local conditions in the nozzle, therefore the presence of pigments can block the nozzle or otherwise modify the local nozzle conditions or block the nozzle such that droplets are not correctly ejected.
Electrostatic ink jet can be characterised by droplets being drawn from an orifice under the influence of an electrostatic field. This field acting between a valving electrode and the orifice, attracts free charges within the ink to its surface such that a droplet is produced when the electrostatic pull exceeds the surface tension of the ink. As this technique relies on attraction of free charges, it therefore requires that the ink be conductive.
A new electrostatic ink jet printing technology has been described in U.S. Pat. No. 6,260,954 to Lima-Marques. This process provides a means of producing variable sized droplets that contain a high concentration of particulate material. Specific advantages conveyed by this process include the ability to form droplets as small as a few micrometers while still using pigments as the colorant material. This is because the size of the droplets are controlled primarily by the voltage on an ejection point plus the ability of the particles to be charged. Also the colorant material is significantly concentrated in the ejected droplets. Therefore high resolution and high density images based on light and water resistant pigments can be produced.
It will be understood by those skilled in the above described non-impact printing art that the operational requirements for an effective liquid toner or liquid developer and that of an ink jet ink, in particular an electrostatic ink jet ink, can be significantly different. For the purpose of this specification and invention, however, the usage of the term marking liquids will be deemed to mean both liquid developers and liquid ink jet inks and in many instances, as would be understood by those skilled in the art, specific references to liquid developers may also be applicable to liquid ink jet inks, in particular to electrostatic ink jet inks.
In general, the process of production of electrostatic marking liquids commences with a resin or a resin system which can contain a resin or a combination of resins and which may also contain a colourant, which can be ground, extruded from a suitable mixing machine or otherwise combined by other techniques known to the art, including means of producing a Masterbatch such as for example a twin roll mill. Additionally included in the resin system there can be added dispersing resins, plasticisers or varnishes, as is generally known in the art.
Additionally, charge directing agents are usually included in the marking liquids to control the polarity and charge to mass ratio of the toner particles. The colourant can be a dye which is soluble in the resin or a pigment comprising of colourant particles which are not soluble in the resin. The resin system and colourant are then milled in a carrier liquid in which neither the resin nor the colourant is soluble, to produce a marking liquid with very fine marking particles distributed in it.
Liquid developers have generally utilized low viscosity liquids and low concentration of the solids content, that is, of marking particles. These traditional toners and associated process systems may be termed low viscosity toner or LVT systems. Generally, LVT systems utilise toners with low viscosities, typically 1 to 3 mPa·s. and low volumes of solids, typically 0.5 to 2% by weight. Maintaining a uniform dispersion of the marking particles can be difficult in a low viscosity toner system. The marking particles have a tendency to drift and settle in the carrier liquid. Furthermore, low volume of solids in the toner increases the amount of toner required to develop a given latent image. More toner will have to be transferred to the photoconductor in order to provide sufficient marking particles for a desired image density.
To overcome these and other known problems that can be associated with LVT systems, highly concentrated liquid toner development systems utilising toner concentrations of up to 60% by weight and viscosities of up to 10,000 mPa·s, and utilizing thin films, typically 1 to 40 μm, of the highly concentrated and viscous liquid toner have been disclosed. This system of developing electrostatic latent images with these viscous and highly concentrated liquid toner systems may be termed high viscosity toner or HVT systems. Examples of such liquid toners are disclosed in commonly assigned U.S. Pat. No. 6,287,741 to Marko, the disclosure of which is totally incorporated herein by reference. Examples of high viscosity, high concentration liquid developing methods and apparatus are disclosed in commonly assigned U.S. Pat. No. 6,137,976 to Itaya et al. and U.S. Pat. No. 6,167,225 to Sasaki et al., the disclosures of which are totally incorporated herein by reference.
Many such hitherto produced marking liquids have been found to have rheologies which can have non-Newtonian flow with applied shear and hence may not have ideal flow characteristics suitable under all conditions for their ultimate intended use. Particle size distribution of the marking particles of so produced marking liquids has also been found to be variable. Other possible problems encountered can include poor dispersion stability, variable electrical characteristics and generally variable print performance.
It is the object of this invention to provide a process for producing these marking liquids which will overcome these problems by providing an alternative ink or toner preparation method, or a post-production method to improve the performance of prior art marking liquids and a marking liquid so produced.